Process control systems, like those used in chemical, petroleum, or other processes, typically include pipes through which the flow of fluid or gas is adjusted by opening or closing valves. The valves are controlled by one or more process controllers communicatively coupled to one or more field devices via analog, digital, or combined analog and digital signal transmission links called buses. The field devices may be, for example, valve positioners, switches, and transmitters (e.g., transmitters of information from sensors of temperature, pressure, fluid level, flow rate, and valve stem position). The field devices are located within a process plant environment and perform process functions such as opening or closing valves, measuring process parameters, gathering diagnostic data, etc.
The process controllers, which may or may not be located within the process plant environment, receive signals representing process measurements made by the field devices, and/or other information pertaining to the field devices. One or more of the process controllers may execute a controller application that runs, for example, different control modules that: (a) make process control decisions, (b) generate control signals based on the received information, and/or (c) coordinate with control modules that are performed by processors located in the field devices. The control modules in the controller send the control signals over the transmission links to the field devices to thereby control the operation of the process.
Information from the field devices and the controller is usually made available over a communication link to one or more other hardware or software devices, such as operator workstations, personal computers, data historians, report generators, centralized databases, etc., which are typically placed in control rooms or other locations away from the harsher plant environment. These hardware devices run applications that may, for example, enable an operator to perform functions with respect to the process, such as changing settings of the process control routine, modifying the operation of the control modules within the controller or the field devices, viewing the current state of the process, viewing alarms generated by field devices and controllers, simulating the operation of the process for the purpose of training personnel, testing the process control software, keeping and updating a configuration database, etc., or testing or gathering data about any of the devices of the process control system, such as any type of valve used in the process control system.
A valve used in the process control system conventionally comprises, as components, a valve seat and a valve closing element that engages the valve seat to close the valve. When these components engage properly, there is a proper valve closure, and the valve has a satisfactory valve seating integrity. Through repeated use in operations of the process control system the valve components may deteriorate due to normal wear, erosion, corrosion, etc.
Traditional smart valve positioners rapidly saturate the servo based on the command signal crossing a cutoff threshold, creating an undesirable valve response, especially on large actuators with volume boosters. For large valves, stem travel can lag significantly behind a servo set-point, especially when tracking ramp signals. When the servo set-point goes into the cutoff threshold, the valve microcontroller and/or positioner will bypass feedback control and drive the current-to-pneumatic (I/P) signal to fully saturate the pneumatics. Cutoffs are based on the servo set-point signal crossing a defined threshold, which is typically set at 0.5% and/or 99.5%. While a fully saturated I/P drive is a desirable state that maintains full seat load in the presence of calibration shifts, for large valves or valves in high pressure service, this can cause the valve plug or actuator piston to hammer into a travel stop, disturbing the process or damaging components.
A mechanism to decelerate the response of the valve as it approaches an end point may typically include a mechanical air cushion and/or an electronic soft stop. Unfortunately, each of these potential implementations includes one or more drawbacks. The mechanical air cushion traps air between the actuator piston and the cylinder end cap. In short, the mechanical air cushion blocks air flow in the exhaust direction, builds up cylinder pressure, and slows down the valve response. To fill the cylinder quickly and distribute air across the piston, additional check valves in the cylinder cap and grooves of the piston face may be required. In addition, the mechanical air cushion is not available for all actuator designs, nor can the mechanical air cushion be field retrofitted. For large valves with heavy moving parts (e.g., plug, stem, and actuator piston), pressure buildup in the cylinder can blow out the air cushion O-ring seals. Also, the mechanical air cushion typically engages near the physical limits of cylinder travel, taking effect around 3% and 97% of rated valve travel. Actuator dynamics change substantially around these areas, which makes throttling control within the 3% threshold problematic. Lastly, because the mechanical air cushion interferes with cylinder pressure registration at the positioner, the accuracy of valve diagnostic tests near the travel endpoints is degraded.
An electronic soft stop is capable of overcoming many of the issues associated with mechanical air cushions. However, the electronic soft stop does not smoothly transition the valve into a hard stop and cannot be used to generate useful diagnostics around the valve seat. Moreover, the electronic soft stop includes several jump discontinuities that can introduce chattering or other undesirable behavior if the process controller is operating near the electronic soft stop threshold, which may lead to an unpredictable response. As such, the electronic soft stop is sensitive to travel calibration errors, which may cause the electronic soft stops to be implemented too early or too late in the dynamic response.
A need therefore exits for providing a controlled saturation of the I/P drive with a smooth, continuous transition of the set-point signal from a cutoff threshold to the saturated state. Controlling a valve with such capability will avoid sensitivities to travel calibration errors, stuck or jammed valves, and will not chatter if the process controller is operating near a hard stop.